Heretofore, practical monolithic caisson-form breakwaters have been constructed at may locations around the world, comprised of horizontally-extended arrays of box-form concrete bodies closed at their bottom by horizontal slabs, each having a seaward-facing vertical front wall that is extensively perforated and spaced from a parallel unapertured rear wall, and defining therewith an upwardly-open container or chamber. Unlike historic bulwarks, moles, seawalls and similar massive masonry piles intended to oppose and reflect most of the incident wave energy, the undermining and catastrophic disintegration which such prior forms experience is wholly avoided by the monolithic caisson, because only a minor part of the energy of incident waves is reflected. Dynamic pressures at seabed adjacent the perforated wall are only slightly greater than if no obstacle whatsoever were encountered by arriving waves; consequently, such scouring of bottom materials by currents as may occur in severest storms is minor, and may easily be rendered harmless by covering the seabed at the toe by a shallow rubble layer.
A full description of such monolithic prior art breakwaters will be found in U.S. Pat. No. 3,118,282 issued Jan. 21, 1964 to Gerard E. Jarlan, and hence need not be repeated here in detail. For convenience and to assist in understanding the present invention, the following brief review is included.
The front wall of a modern caisson-form breakwater presents a large multiplicity of uniformly-distributed openings to the sea, these being formed by the ends of tubular transverse passages which are preferably of cylindric form and of length roughly equal to their diameter, each dimension being of the order of a meter. Wave energy is converted from periodic rising and falling of the sea surface and attendant orbital motion of water particles, into massive horizontal flow through the wall as guided horizontal jets which have aggregate kinetic energy equal to the greater part of the average energy of the wave. The multiplicity of directed jets alternately flowing into the chamber as the sea rises, followed by an equivalent mass outflow as the sea recedes, function as an efficient hydraulic phenomenon requiring only a small head difference to set up the flow, and hence to convert wave energy with only a small reflected component. Other phenomena attending the filling and emptying of the chamber, such as vigorous aeration by spill flow through air/water interfaces, and massive injection into the wave trough, contribute to wave dampling by setting up a zone near the front wall in which turbulence is severe, thereby disordering incident following waves. It is estimated that the reflected energy, expressed as a coefficient of incident wave amplitude, is about 0.14 at the upper part of the front wall, increasing generally linearly downwardly to about 0.21 at the slab bottom.
Important advantages are to be gained in the construction of the box-forms, which proceeds by upwardly advancing slip-forming of the walls above a floating bottom slab, using a shuttering system in a sheltered body of water connected with the open sea by a deep channel which is closed by sea gates, when the caisson height is restricted according to the invention. The construction site does not have to be of an inordinate depth as would be necessary if the minimum floating height is, say, 15 meters. Unlike the procedure attending the casting of walls higher than about 25 meters, where the partially-constructed body must be moved into the sea for the greater part of its building, considerable cost is saved when the structure may be virtually finished in a relatively shallow inlet or embayment. Thereafter, the towing, positioning, and accurate settling of the caissons in alignment is much less difficult than when the structure has a larger vertical extent.